The ability to determine the 3D structure of small objects is of value in a variety of applications, including intra-oral or dental imaging. Intra-oral imaging presents a number of challenges for detecting 3-D structure, such as those relating to difficulty in access and positioning, optical characteristics of teeth and other features within the mouth, and the need for precision measurement of irregular surfaces.
A number of techniques have been developed for obtaining surface contour information from various types of objects in medical, industrial, and other applications. Optical 3-dimensional (3-D) measurement methods provide shape and depth information using images obtained from patterns of light directed onto a surface. Various types of imaging methods generate a series of light patterns and use focus or triangulation to detect changes in surface shape over the illuminated area.
Surface contour imaging uses patterned or structured light and triangulation to obtain surface contour information for structures of various types. In contour imaging, a pattern of lines or other features is projected toward the surface of an object from a given angle. The projected pattern from the surface is then viewed from another angle as a contour image, taking advantage of triangulation in order to analyze surface information based on the appearance of contour lines. Phase shifting, in which the projected pattern is incrementally spatially shifted for obtaining additional measurements at offset locations, is typically applied as part of surface contour imaging, used in order to complete the contour mapping of the surface and to increase overall resolution in the contour image.
Surface contour imaging using structured light has been used effectively for solid, highly opaque objects and has been used for characterizing the surface shape for some portions of the human body and for obtaining detailed data about skin structure. However, a number of technical obstacles have prevented effective use of contour projection imaging of the tooth. One particular challenge with dental surface imaging relates to tooth translucency. Translucent or semi-translucent materials in general are known to be particularly troublesome for patterned light imaging. Subsurface scattering in translucent structures can reduce the overall signal-to-noise (S/N) ratio and shift the light intensity, causing inaccurate height data. Another problem relates to high levels of reflection for various tooth surfaces. Highly reflective materials, particularly hollowed reflective structures, can effectively reduce the dynamic range of this type of imaging.
From an optical perspective, the structure of the tooth itself presents a number of additional challenges for structured light projection imaging. Teeth can be wet or dry at different times and along different surfaces and portions of surfaces. Tooth shape is often irregular, with sharp edges. As noted earlier, teeth interact with light in a complex manner. Light penetrating beneath the surface of the tooth tends to undergo significant scattering within the translucent tooth material. Moreover, reflection from opaque features beneath the tooth surface can also occur, adding noise that degrades the sensed signal and thus further complicates the task of tooth surface analysis. Not all light wavelengths can be detected with equal accuracy. Thus, a multi-spectral or multicolor approach can be less satisfactory in some cases.
One corrective measure that has been attempted is application of a coating that changes the reflective characteristics of the tooth surface itself. To compensate for problems caused by the relative translucence of the tooth, a number of conventional tooth contour imaging systems apply a paint or reflective powder to the tooth surface prior to surface contour imaging. This added step enhances the opacity of the tooth and eliminates or reduces the scattered light effects noted earlier. However, there are drawbacks to this type of approach. The step of applying a coating powder or liquid adds cost and time to the tooth contour imaging process. Because the thickness of the coating layer is often non-uniform over the entire tooth surface, measurement errors readily result. More importantly, the applied coating, while it facilitates contour imaging, can tend to mask other problems with the tooth and can thus reduce the overall amount of useful information that can be obtained.
Even where a coating or other type of surface conditioning of the tooth is used, however, results can be disappointing due to the pronounced contours of the tooth surface and inherent difficulties such as angular and space limitations. It can be difficult to provide sufficient amounts of light onto, and sense light reflected back from, all of the tooth surfaces. For example, different surfaces of the same tooth can be oriented at 90 degrees relative to each other, making it difficult to direct enough light for accurately imaging all parts of the tooth.
A number of problems complicate mapping of an illumination array to sensor circuitry for accurate surface contour measurement. Because multiple images must be captured with the teeth in the same position, any type of movement of the camera or of the patient can complicate the measurement task or require re-imaging and additional measurement and computation time. Thus, it is advantageous to reduce the number of images and amount of time needed for accurate mapping. At the same time, however, measurement improves when multiple images can be obtained and their respective data correlated. Given these conflicting considerations, it can be seen that there are advantages to more efficient pixel mapping techniques that obtain a significant amount of surface contour data from a small number of images.